The present invention generally relates to Air Separation Modules (ASM) and, more particularly, to tubesheets for ASMs.
ASMs have been used in gas generating systems, such as On Board Inert Gas Generating Systems (OBIGGS). Aircraft have used OBIGGS to protect against fuel tank explosions by replacing the potentially explosive fuel vapor/air mixture above the fuel in the ullage space of the tanks with nitrogen enriched air (NEA). The OBIGGS may pass air (e.g. bleed air) through the ASMs, generating the NEA and a stream of oxygen enriched air (OEA). The resulting NEA can be used to inert fuel tanks while the OEA can be recaptured or vented overboard.
The ASM can include a housing having an inlet, an OEA outlet and a NEA outlet. The ASM may include a bundle of hollow fiber membranes positioned within the housing. At least one end of the fiber bundle may be cast or potted in what is commonly referred to as a tubesheet. More commonly, both ends of the fiber bundle may be so encapsulated.
The ASM may be based on permeable membrane (PM) technology. Air at a certain temperature and pressure may enter the housing through the inlet and pass through the bundle of hollow fiber membranes. Oxygen may be separated from the air flow due to diffusion through the fiber walls because the fiber walls may be more permeable to oxygen than to nitrogen. As the air flow travels through the ASM, the NEA flow may be generated by the loss of oxygen via permeation through the fiber wall. The tubesheet(s) may serve to hold the fibers in a fluid-tight relationship such that the NEA flow may be isolated from the OEA flow. The NEA flow may exit the housing through the NEA outlet and the OEA flow may exit through the OEA outlet.
For some ASMs, the operating temperature may be between 140° F. and 220° F. and the pressure difference between the two sides of the tubesheet may be 20 to 60 psig. During use, the module cycles between the operating temperature and ambient every flight. The temperature fluctuation may result in tubesheet cracks after a few thermal cycles. Once cracked, air starts leaking through the cracks, bypasses the membrane and mixes with the NEA, and therefore the desired concentration of nitrogen cannot be achieved.
As can be seen, there is a need for improved tubesheets. Further, tubesheets having a reduction in tubesheet cracking due to thermal cycles are needed. Tubesheets are needed that can improve the separation selectivity of ASMs by reducing tubesheet leakage.